It’s easy to understand why solar power is so popular. It’s clean and abundant, particularly in sunny Arizona. Emission-free solar energy reduces our community’s carbon footprint, helping us combat climate change. Adding solar panels to your home also can reduce your electric bills while adding value to your home.

But some aspects of solar power are harder to understand. Like how it works, when it doesn’t, and why Tucson Electric Power and other utilities cannot rely exclusively on this clean, ubiquitous energy resource to power our communities.

Here are some answers to common questions from customers who are curious about solar power and its expanding role in our energy mix.

How does solar power actually work?

The most common form of solar generation relies on photovoltaic (PV) materials, or cells, that release electrons when they absorb light. PV panels capture those electrons and direct their flow along circuits to create direct current (DC) power. That energy is converted to alternating current (AC) power through an inverter and then carried through conductors to the place where it’s used.

Solar thermal generators, which are less common, rely on the sun’s heat to turn liquid into a gas that can be forced through a turbine to create AC power.

How much energy can solar panels produce?

A typical PV panel can produce between 250 and 400 watts, and system capacity can be increased by adding more panels. A typical residential PV system can produce about 6,000 watts, or 6 kilowatts (kW), while the output of TEP’s larger community-scale systems is measured in millions of watts, or megawatts (MW). TEP’s Wilmot Energy Center includes 314,000 PV panels that can produce up to 100 MW!

Actual production varies depending on how much sunlight is hitting the panels and how efficiently it can be converted into electricity. Typical PV panels convert about 20 percent of the energy in sunlight to electricity, though manufacturers are working to improve the efficiency that can be achieved with commercially available materials.

While efficiency matters, the availability of sunlight is much more important. Solar panels only produce power when the sun is shining on them, and they work best when they’re directly facing the sun. That’s why the PV panels on some larger systems are mounted on motorized racks that keep them pointed toward the sun throughout the day.

But most systems, including those installed on residential rooftops, are installed at fixed angles facing south (at least in the northern hemisphere) to maximize total production throughout the day. This means they’re usually producing much less than their rated capacity.

How many solar panels would I need to power my home?

This one gets asked a lot, but it’s a trick question. The truth is that you can’t actually power a home with solar panels unless you also have electric service – or a whole bunch of batteries.

Electricity is a real-time service that must be provided in the exact amount that’s being used at all times. Solar panels don’t work that way; they just generate as much energy as they can at all times. So homes with solar panels remain fully dependent on the local electric grid for reliable, usable electricity.

It is possible to install enough solar panels to produce as much energy as your home uses over time. But that system will likely produce far more power than you’re using during midday hours while generating less than you use when the sun is lower in the sky – and nothing at all overnight. That system will also remain dependent on the grid.

Batteries can be used to store excess solar energy until it’s needed. But residential storage systems typically aren’t capable of storing enough energy to provide around-the-clock service without support from your local energy provider.

How much solar energy is provided to TEP customers?

Last year, TEP’s solar power systems produced about 877.8 gigawatt-hours (GWh), equivalent to the annual electric use of more than 91,000 typical residential customers. We expect that number to climb this year thanks to our new Raptor Ridge solar array, which was completed earlier this year to provide energy for our GoSolar Home program.

We doubled our clean energy capacity last year with the installation of three large-scale wind and solar power systems, but that’s just the start. We’re planning to add more than 2,400 MW of new wind and solar power systems over the next 13 years to help us provide 70 percent of our power from renewable resources by 2035.

Can TEP power our entire community with solar energy?

It might someday be possible – theoretically, at least – but we shouldn’t. We’ll also want to make use of wind power, another affordable, carbon-free clean energy option that can complement our solar arrays. TEP’s Oso Grande Wind farm usually produces peak output during early morning and evening hours when our solar resources aren’t very productive. That characteristic is typical of other wind farms and helps us balance our energy resources.

Wouldn’t it be better to put rooftop solar across the entire city instead of building big solar installations?

Rooftop solar arrays are great, but they cost much more than large community-scale systems – more than three times as much, according to a recent report by the Solar Energy Industries Association. While we’re happy to help customers who want to install systems of their own, our own systems must be as cost-effective as possible to help us maintain affordable service.

We make it easy for customers to benefit from our cost-effective community-scale systems. While all customers receive some solar energy, the TEP GoSolar Home program lets you buy all your power from a local solar array for a fixed monthly cost. The TEP GoSolar Shares program provides a flexible way to serve some or all of your energy from the sun for an affordable price.

Do solar panels work when it’s cloudy?

PV panels are less efficient on cloudy days, but they usually still produce some power. One reason TEP installs solar arrays at different locations around our city is to make it more likely that some systems will get glimpses of sunlight even if others are covered by clouds.

But clouds aren’t all bad, particularly if they contribute to cooler weather. Solar panels are less efficient in the triple-digit summer heat, producing optimally up to about 95 degrees. There’s even a chance that clouds can increase solar production – for a few moments, anyway – when light passing through the edge of a passing cloud is bent toward panels that are also receiving direct sunlight.

What should I consider before adding a solar power system to my home?

There are a number of factors you should consider, including local building codes, available sunlight, your roof design, whether to lease or own and maintenance and repairs.  The financial aspects of investing in a PV system also should be weighed.

TEP’s Solar Analysis tool can help you determine if installing a PV system makes sense for you. The tool helps you evaluate all of your solar options to determine if a PV system will meet your financial and sustainable goals. This tool estimates the upfront and long-term costs, potential cost saving and payback period, the recommended size of the system for your home and other factors so that you can make an educated decision.

Does TEP provide solar incentives for homeowners?

Yes. The Resource Credit Proxy (RCP) payments we provide for excess solar energy  help make rooftop systems more affordable for our customers. They also greatly exceed the cost of power from larger, community-scale solar power systems, which can be purchased for less than three cents per kilowatt-hour.

This generous incentive is one reason rooftop solar installations have increased to record levels in our service area since the RCP rate was approved by the Arizona Corporation Commission (ACC). TEP interconnected a record 5,200 PV systems last year, and that number is expected to surpass 7,000 in 2022.

How can I find a reputable solar installer?

TEP works collaboratively with local installers, but we don’t endorse or have formal partnerships with them. Beware of companies that claim to work on behalf of TEP or be a “preferred” contractor.

It’s important to do your homework before committing or signing a contract. Here are some helpful tips for choosing a solar company with experience and integrity:

• Hired a licensed contractor. Solar installers must be licensed by the Arizona Registrar of Contractors (AZROC) and possess an Arizona business license that is active and in good standing. Verify the company’s license status by calling 877-692-9762 or visiting the AZROC website. Your installer also should be insured to protect yourself against liability for injuries on your property.
• Make sure they are familiar with TEP’s requirements and the application process. Look for installers that have completed other PV system installations in our service area or have a solid working knowledge of the requirements, to help ensure a successful installation.
• Get several proposals with detailed estimates.  Prices, project specifications and warranties vary significantly. Obtain detailed quotes from several companies and compare them to make sure the proposed system will meet your goals and expectations.
• Ask for references. Request at least three references from customers whose projects are similar to yours and call them. Check the installer’s service record and whether they have any unresolved complaints, judgments or liens. Check their Better Business Bureau rating.
• Get everything in writing. As with any large purchase or contract, any material changes to the system design, performance, equipment or cost should be documented in writing.
• Read all documents carefully before signing. You need to understand the terms and conditions before you sign. This helps prevents disagreements during and after installation. TEP advises against signing a contract or making payments on leased systems until TEP installs a DG meter and issues a Permission to Operate.

Here are some suggested questions to ask prospective solar installers before you hire one.

Q: How will a private solar array impact my monthly bill from TEP?

A: This is one of the key questions your installer can help you answer. You should expect your bill to decrease, because you’ll be producing some of your own energy. Those savings will likely increase over time as TEP’s rates rise, but check to see how much of an increase is built into your calculation. Our rates have been relatively stable over time, increasing at about 1.2 percent annually, on average, over the past 25 years.

Q: Will my private solar array generate power during a grid outage?

A: If a power outage occurs, your system is required to shut down automatically for safety reasons. This prevents excess energy generated by your system from flowing back to our circuits while our crews are repairing equipment, which must be de-energized to prevent an electrical hazard. Once power is restored, you can resume operation of your system, which may require a manual reset.

Customers whose private solar arrays are coupled with a battery storage system and a “smart switch” may be able to use solar energy to power their home during an outage. The smart switch senses the loss of power from TEP and automatically isolates the customer’s PV system from TEP’s grid. Once isolated, the PV system can continue to produce energy if the sun is shining, and the battery energy can be used. Customers with larger capacity batteries may have enough stored energy to provide “whole house backup power” for several hours.

Q: Can I still use solar energy if I can’t install panels at my home?

A: Yes! Solar energy is included in the service we provide all customers, and the amount of solar energy we provide will increase steadily over time. But if you want more, TEP’s GoSolar Home and GoSolar Shares programs offer an affordable way to tap the clean, renewable energy produced by our community-scale solar arrays.